1. Field of the Invention
The present invention relates to polyethylene glycol-carbon powder system heat sensitive-electrically resistant composites which are capable of performing the self-temperature control by showing the steep increase in electrical resistance at certain temperatures when they are used as plane heaters, and also to a method for controlling the steady state exothermic temperature for the foregoing composites.
2. Prior Art
The detailed proposals with respect to polyalkylene oxide-carbon particle system self-temperature controlling exothermic composites, which constitute the basis of this invention, have already been made by the inventor in the Japanese laid-open Pat. No. 1984-110101 and No. 1985-140692 (U.S. Ser. No. 649,535). When power is supplied, the temperature of these composites increases, and when the temperature reaches a point above a certain temperature, the value of the electric resistance of these composites increases sharply thereby causing a sharp decrease in current.
On the contrary, when the temperature of the composites is brought down by cooling the composites from the outside, the value of electric resistance decreases and the current increases. Making use of this property, heaters can be obtained wherein the composites are maintained at constant temperatures. The electrically resistant mentioned above are called PTCR (Positive Temperature Coefficient Resisters). Conventionally, barium titanate has been known as an inorganic PTCR, while polyethylene-carbon particle mixed systems have been known as an organic PTCR.
However, the barium titanate heater is disadvantageous in that it is impossible for it to bring the steady state exothermic temperature down to 70.degree. C. or below; and also in that it is not appropriate for obtaining a plane heater having a large area because it is a sintered body. Besides, it is high in price. Also, its effect as a PTCR is not as high as polyethylene oxide-carbon particle mixed system. However, the polyethylene (paraffin)-carbon particle system has a compatibility problem. Therefore, it is still being studied.
The foregoing polyalkylene oxide-carbon particle system self-temperature control heater (i.e. exothermic) composite is highly stable, and it is a new heater with which high PTCR effect can be effected. Furthermore, its steady state exothermic temperature has the feature that, as long as the concentration of the carbon particles stays within a certain range, it remains almost constant even when the concentration varies to some minor extent. When the concentration of the carbon particles is lowered to a point below the foregoing range, the exothermic temperature begins to vary greatly depending on proportional to the concentration. However, when viewed from the standpoint of practical application, it is not desirable to use such a composite, in terms of quality control, because a slight variation in the amount of the carbon particles (for example, graphite) causes substantial differences in the electric characteristics of the products.
The inventor has further confirmed that, as a technique to stabilize and to optionally control the steady state exothermic temperature, when a third material, such as water or, alcohol, is added, the above mentioned stabilization and optional control become feasible (Japanese laid-open Pat. No. 1986-39475).
However, the means and the composite which makes it possible to set the steady state exothermic temperature at a preferred temperature within the same composite without adding a third material, etc. as mentioned above, have not yet been found.